Plasma arc torches are commonly used for the working of metals, including cutting, welding, surface treatment, melting, and annealing. Such torches include an electrode which supports an arc which extends from the electrode to the workpiece in the transferred arc mode of operation. It is also conventional to surround the arc with a swirling vortex flow of gas, and in some torch designs it is conventional to also envelop the gas and arc with a swirling jet of water.
The electrode used in conventional torches of the described type typically comprises an elongate tubular member composed of a material of high thermal conductivity, such as copper or a copper alloy. The forward or discharge end of the tubular electrode includes a bottom end wall having an emissive element embedded therein which supports the arc. The element is composed of a material which has a relatively low work function, which is defined in the art as the potential step, measured in electron volts (ev), which permits thermionic emission from the surface of a metal at a given temperature. In view of its low work function, the element is thus capable of readily emitting electrons when an electrical potential is applied thereto, and commonly used emissive materials include hafnium, zirconium, tungsten, and their alloys.
A significant problem associated with torches of the described type is the short service life of the electrode, particularly when the torch is used with an oxidizing gas such as oxygen or air. More particularly, the gas tends to rapidly oxidize the copper of the electrode which surrounds the emissive element, and as the copper oxidizes its work function decreases. As a result, a point is reached at which the oxidized copper surrounding the emissive element begins to support the arc, rather than the element. When this happens, the copper oxide and the supporting copper melt, resulting in early destruction and failure of the electrode.
The assignee of the present application has previously developed an electrode with significantly improved service life, as described in U.S. Pat. No. 5,023,425, the entire disclosure of which is hereby incorporated herein by reference, and a method for making such an electrode, as described in U.S. Pat. No. 5,097,111, the entire disclosure of which is hereby incorporated herein by reference. The '425 patent discloses an electrode comprising a metallic tubular holder supporting an emissive element at a front end thereof, and having a relatively non-emissive separator or sleeve surrounding the emissive element and interposed between the emissive element and the metallic holder. The sleeve thereby separates the emissive element from the holder. The '425 patent describes the sleeve as preferably being formed of silver which has a high resistance to formation of an oxide. The silver and any oxide thereof which does form are poor emitters, and therefore, the arc will continue to emit from the emissive element rather than from the sleeve or the metallic holder. Service life is thereby significantly extended. The sleeve has an end face flush with the ends of the holder and emissive element, the end face in one embodiment being defined by a radially outwardly extending annular flange portion of the sleeve.
The '111 patent discloses a method for making an electrode which includes the steps of forming a counterbored cavity in the front face of a cylindrical blank of copper or copper alloy, the cavity including an annular outer end portion for receiving the annular flange portion of a non-emissive member. A second metal blank of relatively non-emissive material, preferably silver, is formed to substantially fit within the cavity. The non-emissive blank is then metallurgically bonded into the cavity by first inserting a disk of silver brazing material into the cavity, then inserting the non-emissive blank. The assembly is then heated to a temperature only sufficient to melt the brazing material, and during the heating process the non-emissive blank is pressed into the cavity, which causes the brazing material to flow upwardly and cover the entirety of the interface between the non-emissive blank and the cavity. The assembly is then cooled, resulting in the brazing material metallurgically bonding the element into the non-emissive blank. Next, the non-emissive blank is axially drilled and a cylindrical emissive element is force fitted into the resulting opening. To complete fabrication of the electrode, the front face of the assembly is machined to provide a smooth outer surface which includes a circular outer end face of the emissive element, a surrounding annular ring of the non-emissive blank, and an outer ring of the metal of the holder.
Published Japanese Patent Application No. 4-147772, filed on Oct. 8, 1990 and published on May 21, 1992, describes a plasma arc torch electrode having a copper holder and a cylindrical function insert for supporting an arc, and a metal spacer disposed between the function insert and the holder for establishing thermal and electrical coupling therebetween. As is conventional in plasma arc torches, the holder is cooled by circulating a coolant through the interior of the holder. The patent application describes as an object of the metal spacer to increase the thermal transfer ratio between the holder and the function insert so that improved cooling of the function insert can be attained, which is said to increase the life of the electrode. The metal spacer consists of a hollow cylindrical member open on both ends and surrounding the cylindrical function insert. The metal spacer in one embodiment is composed of a silver alloy containing 24-95 percent silver and 5-74 percent copper. This alloy is said to accomplish the goal of achieving a lower melting point for the metal spacer than for the holder and the function insert, such that the metal layer between the function insert and the holder melts before either of those members and flows between them, thus protecting the holder from the plasma arc and absorbing the heat from the tip end of the function insert by the latent heat of evaporation. The copper content of the alloy is said also to facilitate diffusion bonding both with the copper holder and with the emissive element which is composed of hafnium or an alloy thereof, or zirconium or an alloy thereof. The patent application states that the radial thickness of the metal spacer should be 0.01-0.8 mm, and that greater thickness is undesirable because then the whole metal layer of the spacer can melt and allow the function insert to fall out of the holder.